Mass spectrometry



April Z, 1946.

R. c. oLEsEN 2,397,560

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27 QfGuL/:rso i 26 ,e c. C//ecu/ r [25) IN VEN TOR. RA YMO/V C. 'SEN Patented Apr. 2, 1946 MASS SPECTROMETRY Raymond C. Olesen, Altadena, Calif., assigner to Consolidated Engineering Corporation, Pasadena, Calif., a corporation of California Y Application August 18, 1943, Serial No. 499,055

(Cl. 'I3-18) 10 Claims.

This invention is concerned with mass spectrometers and particularly with mass spectrometers of the type in which a plurality of diverging ion beams are swept successively over an ion collector. It provides improvements in beam sweep mechanisms which aiord economies in construction and improved operation.

In the analysis of mixtures by mass spectrometry, the mixture, which conveniently is in vapor phase, is introduced into the instrument and subjected to ionization, usually by bombardment with ionizing particles such as electrons. Molecules in the mixture are thus converted to ions, which are propelled along a path in an analyzer chamber (which ordinarily ls maintained under high vacuum) toward a collector. In the analyzer, an electrical or magnetic field or both acts upon the ions and separates them according to their mass-to-charge ratio, i. e. their specic mass, thus separating the heterogeneous ion beam into a plurality of diverging and usually curved beams of ions having different speciic masses. By altering the propelling potential or the analyzer field or both, the general direction of the'beams may be changed, so that they impinge successively upon an ion collector. The currents corresponding to the several beams are measured and afford both a qualitative and quantitative determination of the kinds of ions striking the collector. For example, under proper operating conditions a mass spectrometer may be employed to analyze both quantitatively and qualitatively a complex mixture of hydrocarbons.

In one method of analysis, the propelling ptential or the analyzer eld (electrical or magnetic) is altered slowly, progressively and automatically to bring about a proper sweep of the ion beams. Several devices for this purpose are disclosed and claimed in the copending application Serial No. 444,491, led May 25, 1943, by Robert V. Langmuir. Some of these devices. While satisfactory from many standpolnts, require high operating potentials and currents at the point of control, with consequent expensive forms of condensers, etc., and some operating hazard.

As a result of my investigation, I have develf oped an improved beam sweep mechanism which overcomes the above-noted diiiiculties and which aords further advantages described hereinafter. Although it may be employed to control the beam sweep by varying the analyzer field, it is particularly useful for varying and controlling the pro- 5 peiling potential and is described in detail hereinafter in such an application.

As indicated above, the control mechanism of my invention is adapted for use in a mass spectrometer provided with means for converting molecules into ions, an analyzer, a collector, means for impressing a potential upon the ions to propel them through the analyzer towards the collector, means for producing a eld (electrical or magnetic or both) in the analyzer to separate the ions according to their specic masses, whereby a`plurality of diverging beams of ions having different specific masses may be formed in the analyzer, and means for altering the paths of the beams in the analyzer by varying the propelling potential or the analyzer field or both, whereby the beams of ions of different specific masses may be directed successively on the collector.

In such an apparatus my invention contemplates the combination which comprises a main circuit operatively connected to the path altering means, a D. C. voltage supply (preferably substantially constant) connected to the main circuit, a vacuum tube connected in the main circuit and provided with a grid, a resistancecondenser (RC) circuit connected to the grid, and an auxiliary D. C. power supply connectable to the R-C circuit for charging its condenser. Thus the R-C circuit is connected in controlling relationship to the main circuity through the vacuum tube and controls the voltage applied across the main circuit by control of the resistance of the vacuum tube.

Conveniently the apparatus is provided with a switch for cutting the second D. C. power supply `out o the R-C circuit, so that this power supthe light of the following detailed description taken in conjunction with the accompanying drawings in which:

Fig. 1 is a diagram of a simplified mass spectrometer equipped with a beam sweeper of my invention;

Fig. 2 is a wiring diagram illustrating a preferred form of the beam sweeper of my invention; and

Fig. 3 illustrates another arrangement of the beam sweeper according to my invention.

Referring now to Fig. 1, it will be observed that the apparatus comprises an ionization chamber I followed by a substantially semi-circular tubular analyzer Il, both disposed between the poles of a large electromagnet (not shown) and adapted to be maintained at low pressure, for example, by enclosure in an evacuated envelope. A sample of gas to be analyzed is admitted into the ionization chamber through an inlet I2 and is there bombarded by an electron beam I3, so that molecules of the gas sample become ionized. 'I'he resulting ions are pushed out of the ionization chamber by an electrical potential impressed between a pusher electrode P disposed within the ionization chamber and a rst propelling electrode I4 which has an exit slit S1 through which the ions pass. The ions leaving the ionization chamber are further accelerated by means of a potential impressed between the rst propelling electrode and a. second propelling electrode I5 which' is providedwith a slit S2 matching the slit S1. In this fashion, a heterogeneous ion beam I6 is propelled through the slits, which act as a col` limator. Within the analyzer tube the ions, under the innuence of the magnetic field, tend to pursue curved paths. Thus, the ions of one specic mass pursue a path of one radius, while ions of a diierent specic mass pursue a path of a different radius, the result being a plurality of separated homogeneous ion beams which diverge from each other. there is an exit slit I l. A homogeneous beam which is focused on this slit passes through it and strikes anion collector I8. The current thus generated in the ion collector is amplied and recorded to produce a mass spectrogram.

In order that the several separated ion beams may be collected and measured, it is desirable to cause the beams successively to become focused on the exit slit. 'Ihis is accomplished by gradually varying the accelerating potentials applied to the ion accelerating electrodes P, I4, I5. In the instant case, this is accomplished by gradually decreasing the accelerating potentials.

As shown in Fig. 1, the apparatus is provided with a regulated high voltage direct current power supply 20, the output of which may be very high, say, of the order of 4000 volts. The output of the power supply is applied, respectively, to the electrodes through a potential divider 2l comprising a small resistance 22 and a larger resistance 23 connected in series. The pusher electrode is shunted in this circuit ahead of the smaller resistance. The first propelling electrode is shunted between the two resistances and the second propelling electrode is connected to the opposite end of the larger resistance. In this way, a relatively small pusher potential is impressed between the pusher electrode and the rst propelling electrode while a larger potential is impressed between the two propelling electrodes I4,

A vacuum tube 2d is connected across the high voltage power supply. The grid 24a of this tube is connected to an R-C circuit 25 comprising a condenser 25A and a resistance 25B in parallel with each other. An auxiliary direct current power supply 26 is connected in parallel with the R-C circuit. a switch 2l being connected between the resistance 25B of the R-C circuit and the auxiliary power supply. The circuit is also pro- At the end of the analyzer vided with resistors 30, 3I connected in one side of the circuit respectively, between the power supply and the tube and between the tube and the potential divider. Y

In the apparatus described there is a large negative voltage applied to the grid and across the condenser when the switch 2l is closed. Consequently, the tube resistance is high' and a high voltage is impressed across it. If, thereafter, the switch is opened the condenser will discharge with a corresponding decay in the resistance of the tube and of the voltage in the circuit. 'Ihe voltage applied to the ion accelerating electrodes Will then decrease gradually, bringing about a sweep of the ion beams successively across the exit slit to impinge upon the collector, the result being the recording of mass spectrogram in the form of a series of current peaks, each correspending to an ion beam of a given specic mass.

Several advantages reside in th'e use of a vacuum tube to control potential decay in accordance with my invention. In the apparatus just described, the decay is controlled fundamentally by ther condenser and because of the use of the vacuum tube,y the same decay can be obtained with lower voltages applied to the condenser. Consequently, cheaper condenser constructions can be employed. A further advantage resides in th'e fact that the lower potentials are less hazardous.

In Fig. 2, I have illustrated a preferred form of my beam sweeper, which is adapted to be conm nected to a mass spectrometer through a poten tial divider as in the apparatus of Fig. l.

Referring to Fig. 2, it will be observed that I provide the high voltage regulated power supply 20 connected through a beam sweeper 40 to a potential divider 4I, which in turn is connected to the ion accelerating electrodes of a spectrometer, as shown in Fig. 1. Thus, the high voltage of the regulated lpower supply is applied successively through resistances 3|, 30 to the potential divider. A triode 42 capable of operating at high voltages acts as a variable resistance and is connected between the ground terminal 43 of the regulated power supply and a junction 44 between the resistances 3|, 30. Thus, the anode of the triode is connected to the junction. The filament of th'e triode is connected to ground through a sliding contact 45 on a resistance 46 connected across the secondary 41 of a filament transformer 48.

This filament transformer may be connected through a switch 49 to a Sola regulator.

A resistor R and a condenser C are connected in parallel between the grid of the triode and ground, thus forming an R-C combination.

If desired, a large condenser (not shown) may be connected across the output terminals of the beam sweeper in order to filter out any residual ripple appearing at this output due either to slight iiuctuations in the voltage in the regulated power supply or to variation in the temperature of the filament of the vacuum tube.

D. C. voltages may be applied across the parallel R-C circuit from a voltage selector circuit 50. This circuit comprises three potentiometers 5 I, 52, 53 which may be connected across a battery 54 or across the output of a voltage regulator 55, or disconnected from both at the will of the operator by means of a single pole triple throw switch 56.

Potentials from anyone of the three potentiometers may be applied to the grid terminal of the condenser C through a switch 51 which has 2,897,560 one spring release contact and one holding contact. This switch is placed in the holding contact position when it is desired to place a constant voltage across the condenser. Itis placed in the spring relay contact position if it is desired to charge the condenser normally preparatory to recording a mass spectrogram.

'I'he bias on the grid of the regulator tube, i. e. the triode 42, is the voltage across the condenser. It is this bias that determines the resistance of the regulator tube which, in turn, determines the voltage across the potential dividing network. A shift in the potential applied to the potential di viding network causes the beams in the analyzer tube to be swept across the exit slit.

On occasion, it may be desirable to hold the beams steady, as when a study of a particular beam is to be made. In such case, a steady voltage is applied to the grid of the regulator tube through the holding contact. On the other hand, if the condenser is charged to some predetermined voltage and then permitted to discharge, as is the case when the switch l is thrown to the spring release contact, ions of different specific mass will impinge successively upon the ion collector as the beams'are swept across the exit slit.

Slight fluctuations occur in the output of the voltage regulator. Hence, application of potential from the voltage regulator to the potentiometers is not desirable when a singlefbeam is to be held stationary at the ion collector. When such steady voltages are to be applied, it is desirable to connect the battery 5B to the potentiometers. On the other and, when the condenser is to be charged and then discharged to sweep the beams successively past the ion collector, the potentiometers may be connected to the output of the voltage regulator.

The voltage regulator is a conventional structure except that its positive terminal is grounded instead of its negative terminal. Power is supplied to the voltage regulator through a rectier 58 that derives its power through a switch 59 from the Sola regulator (not shown).

The system of Fig. 3 is similar to that shown in Fig. 1, except that the regulated D. C. voltage supply is connected with the winding Si) for the analyzer field, thereby producing the same change in voltage on the analyzer field as is produced on the electrodes I 4 and i5 in Fig. 1. The application of this regulated voltage on the analyzer eld has the same effect in sweeping the beam as does the application of the regulated voltage to the electrodes id and l5 in Fig. 1.

I claim:

1. In a mass spectrometer provided with means l for converting molecules into ions, an analyzer, a

collector, means for impressing a potential upon the ions to propel them through the analyzer toward the collector, means for producing a field in the analyzer to separate them according to their specic masses so that a plurality of diverging ion beams each comprising ions of a different specific mass may be formed in the analyzer, and means for altering the paths of the beams in the analyzer by varying the propelling potential, the 4combination which comprises a main circuit operatively connected to the path-altering means, a substantially constant D. C. voltage supply connected to the main circuit, a vacuum tube having a cathode, anode and grid connected as a variable impedance element in the main circuit in controlling relationship therewith, a resistance-condenser circuit connected between the grid and cathode, and an auxiliary D. C. power supply connectable to the resistance-condenser circuit for charging its condenser.

2. In a mass spectrometer provided with means for converting molecules into lons. an analyzer, a collector, means for producing a tleld in the analyzer to separate the ions according to their specic masses, means for impressing a potential on the ions to propel them through the analyzer toward the collector whereby a plurality of diverging beams each comprising ions of a dierent specific mass may be formed in the analyzer, and means for altering the paths of the beams in the analyzer by varying the potential, whereby beams of ions of different specific masses may be sucessively directed onto the collector, the combination which comprises a circuit connected to the path altering means, a substantially constant D. C. voltage supply connected to the circuit, a vacuum tube connected across the circuit said tube being provided with a grid and a cathode, an auxiliary D. C. power supply adapted to be connected between the grid and cathode, and a resistance-condenser circuit connected between the grid and cathode in controlling relationship with the grid, a switch for cutting the auxiliary D. C. power supply out of the resistance-condenser circuit, so that throwing the switch brings about a progressive change in the voltage impressed upon the path altering means, whereby the beams of ions of different specic mass are swept successively past the collector.

3. In a mass spectrometer provided with means for converting molecules into ions, an analyzer, a collector, means for impressing a potential upon the ions to propel them through the analyzer toward the collector, means for producing a eld in the analyzer to separate them according to their specific masses, so that a plurality of diverging homogeneous ion beams of different specific mass may be formed in the analyzer, and means for altering the paths of the beams in the analyzer by varying the propelling potential, the combination which comprises a main circuit operatively connected to the path-altering means, a D. C. voltage supply connected to the main circuit, a vacuum tube connected as an impedance element in the main circuit in controlling relationship therewith, and provided with a grid and cathode, a resistance-condenser circuit connected to the grid-cathode circuit, and an auxiliary D. C. power supply connectable to the resistance-condenser circuit for charging its condenser.

4. In a mass spectrometer provided with means for converting molecules into ions, an analyzer, a collector, means for producing a field in the analyzer to separate the ions according to their specic masses, means for impressing a potential on the ions to propel them through the analyzer toward the collector whereby a plurality of diverging homogeneous beams of ions of different specific mass may be formed in` the analyzer, and means for altering the paths of the beams in the analyzer by varying the potential, whereby beams of ions of diierent specific charges may be successively directed onto the collector, the combination which comprises a circuit connected to the path altering means, a D. C. voltage supply connected to the circuit, a voltage selector connected between the supply and the circuit, a vacuum tube connected across the circuit and provided with a grid-cathode circuit, an auxiliary D. C. power supply, and a resistance-condenser circuit connected in the grid-cathode circuit between the grid and the auxiliary D. C. power supply in controlling relationship with the grid, a switch for cutting the auxiliary D. C. /power supply out of resistance-condenser circuit, so that throwing the switch brings about a progressive change in the voltage impressed upon the path altering means, whereby the beams of ionsvof different speciilc mass are swept successively past the collector.

45. In combination with a mass spectrometer comprising an ion analyzer, a collector and electrical means for propelling a plurality of diverging ion beams through the analyzer to the collector: a source of electrical voltage applied to said electrical means, a vacuum tube having a cathode, anode and grid with its anode-cathode circuit connected across said source, a condenser shunted by a resistance connected in the gridcathode circuit and a sourcev of D. C. voltage adapted to be connected and disconnected from said condenser. Y

6; In combination with a mass spectrometer comprising an ion analyzer, a collector and electrical means for propelling a plurality of diverging ion beams through the analyzer to the col` lector: a potentiometer for supplying a controlling voltage to said electrical means, a source of electrical voltage connected across said potentiometer, a vacuum tube having a cathode, anode and grid with its anode-cathode circuit in shunt with respect to said potentiometer and source, a

. condenser in parallel with a resistance connected in the grid-cathode circuit of said tube, and means for changing the voltage on said grid.

7. In combination with a mass Spectrometer comprising an ion analyzer, a collector and electrical means for propelling a plurality of diverging ion beams through the analyzer to the co1- lector: a source of electrical voltage applied to said electrical means, a resistor in series betweenl said source and said electrical means, a vacuum tube having a cathode, anode and grid with its anode-cathode circuit connected in shunt relation with respect to said electrical means at a point between said means and said resistor, a condenser shunted by a resistance connected in the grid-cathode circuit of said tube, and a source of D. C. voltage adapted to be switched across said condenser.

8. In combination with a mass spectrometer comprising an ion analyzer, a collector and electrical means for propelling a plurality of diverging ion beams through the analyzer to the collector: a potentiometer for supplying a controlling voltage to said electrical means, a source of electrical voltage connected across said potentiometer, resistance means in series between said source and said electrical means, a vacuum tube having a cathode, anode and grid with its anodecathode circuit in shunt relation with respect to said potentiometer at a point between said potentiometer and said resistance means, a condenser shunted by a resistance in the grid-cathode circuit of said tube, and a D. C. voltage source connectible to the grid-cathode circuit to charge the condenser and permit its discharge through the resistance shunted across it.

9. In a mass spectrometer provided with means for converting molecules into ions, an analyzer, a collector, means for impressing a potential upon the ions to propel them through the analyzer toward the collector, means for producing a iieid in the analyzer to separate them according to their specific masses so that a plurality of diverging ion beams each comprising ions of a different speciflc mass may be formed in the analyzer, and means for altering the paths of the beams in the analyzer by varying the analyzer.

field, the combination which comprises a main circuit operatively connected to the path-altering means, a substantially constant D. C. voltage supply connected to the main circuit, a vacuum tube having a cathode, anode and grid connected as a variable impedance element in the main circuit in controlling relationship therewith, a resistancecondenser circuit connected between the grid and cathode and an auxiliary D. C. power supply connectable to the resistance-condenser circuit vfor charging its condenser.

l0. In a mass spectrometer provided with means for converting molecules into ions, an analyzer, a collector, means for producing a eld in the analyzer to separate the ions according to their speciilc masses, means for impressing a.

potential on the ions to propel them through the analyzer toward the collector whereby a plurality of diverging beams each comprising ions of a different specific mass may be formed in the ana lyzer, and means for altering the paths of the beams in the analyzer by varying the field, whereby beams of ions of diierent specific masses may be successively directed onto the collector, the combination which comprises a circuit connected to the path altering means, a substantially constant D. C. voltage supply connected to the circuit, a vacuum tube connected across the circuit said tube being provided with a grid and a cathode, an auxiliary D. C. power supply adapted to be connected between the grid and cathode, and a resistance-condenser circuit connected between the grid and cathode in controlling relationship with the grid, a switch for cutting the auxiliary D. C. power supply out of the resistance-condenser circuit, so that throwing the switch brings about a progressive change in the voltage impressed upon the path altering means, whereby the beams of ions of different speciilc mass are swept successively past the collector.

RAYMOND C. OLESEN. 

